This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to flow independent projection angiography using MRI.
To date, imaging vasculature using magnetic resonance has relied almost exclusively on flow information via the phase distortion of spins moving through a gradient field or the wash-in of spins. These techniques are currently finding an array of clinical applications, particularly in the head and neck. However, in some situations where the clinician is interested in morphological information rather than velocity characteristics in the vessels, the flow sensitivity of these sequences can become a liability.
Consider the demands placed on an MR technique for imaging peripheral vascular anatomy. Such a method must generate high resolution images with a large, volumetric field of view and a high signal-to-noise ratio (SNR) in reasonable examination times. The sequence must suppress signals from all other tissues in the region and be insensitive to artifacts particularly those induced by body motion and blood flow. It should image vessels with a wide range of flow velocities and orientations. Robustness and simplicity are also important features for clinical application.
Phase contract and wash-in methods experience several problems in this application, most notably in imaging the veins and smaller vessels of the peripheral region where flow is inherently slow. To be sensitive to low velocities, phase contrast methods must severely restrict the range of velocities that can be imaged without phase wrap. Doing so increases sensitivity to global motion artifacts as well as to signal loss caused by velocity dispersion within a vessel. Furthermore, the phase contrast methods require multiple acquisitions to image the full range of potential vessel orientations. Wash-in techniques suffer from inadequate vessel filling in the presence of slow flow. This restricts the potential imaging volume limiting the field of view and the SNR of the resultant images. Both phase contrast methods and time-of-flight selective tagging methods are sensitive to time-varying instabilities in the imaging system since they isolate a weak blood signal from a much larger stationary tissue signal by subtracting two signals acquired at different times.
Our goal is to image vascular anatomy, particularly vessels with a slow flow, without the problems of current sequences. By designing a sequence that is insensitive to flow, we minimize the flow-related problems faced by phase contrast and wash-in methods. However, by doing so, we eliminate the availability of flow information for distinguishing vessels from surrounding tissue. We must then rely on other NMR parameters, particularly the unique relaxation characteristics of blood and its chemical shift relative to fat, to isolate the blood-filled vessels. To avoid the artifacts inherent in techniques that combine multiple acquisitions, the sequence should leave only the blood signal present when data is recorded. To meet the stringent resolution, SNR, field of view, and examination time demands of vascular imaging, signals from projections through the volume of interest should be acquired. The challenge is to produce such a sequence that is also both simple and robust.